Sensors are used to monitor parameters of interest in a variety of different applications. For example, in avionic applications, the parameter of interest may be the position of an aircraft's landing gear, flaps, or ailerons. Alternatively, the parameter of interest may be the temperature, pressure, or flow rate of some fluid involved in an industrial process. In a medical application, the parameter of interest may be a patient's temperature, pulse rate, or blood pressure.
Regardless of the application involved, a sensor produces an output that is representative of the parameter of interest. In one class of sensors, referred to herein as optical sensors, the sensor output is based at least partially upon the processing of an optical beam.
More particularly, in a "passive" optical sensor, the sensor modulates an optical input beam to produce an optical output beam that is representative of the parameter of interest. A "hybrid" optical sensor, on the other hand, receives an electrical input signal and produces an optical output beam that is representative of the parameter, or receives an optical input beam and produces an electrical output signal representative of the parameter. Finally, a "self-generating" optical sensor produces an optical output beam in direct response to the parameter, without receiving either an optical or electrical input.
Optical sensors have several advantages over more traditional electrical sensors. For example, the optical beams employed by such sensors are typically not disrupted by electromagnetic fields. Optical sensors are also preferred for use in flammable environments because of their reduced risk of sparking or heating. Further, optical sensors are often relatively compact and lightweight.
Conventional optical sensors do, however, have some limitations. In that regard, the losses involved in the transmission of optical beams used by such a sensor may significantly affect the interpretation of the sensor's output. As will be appreciated, external losses may occur as the result of the absorptive, reflective, and refractive nature of the components used to transmit optical beams to and from the sensor. Similarly, internal losses may be introduced by the components that transmit optical beams within the sensor.
In the past, attempts have been made to limit the influence that optical losses have on the operation of optical sensors. For example, one approach that has been used involves the measurement of the internal losses associated with a particular sensor and the external losses associated with the system the sensor is used in. The measured optical losses are then taken into account when the sensor output is interpreted.
In that regard, the measured losses alter the relationship that would otherwise exist between the sensor output and the parameter of interest. Thus, if the sensor output is normally applied to an equation to compute, for example, the position of an object, the equation must be altered to account for the measured losses. Similarly, if a look-up table is used to determine the position of the object corresponding to a particular sensor output, the look-up table must be altered to account for the losses.
Along with the initial calibration of the sensor and system, another approach has been developed to remove the influence of losses on the sensor's output, without requiring the losses to be measured. More particularly, an optical beam including components having more than one wavelength is transmitted through the elements whose losses are to be accounted for. As will be appreciated, the losses associated with these elements, which form a common path for the beam, affect each of the different wavelength components the same. For that reason, these losses are referred to as "common-mode" losses.
After traversing the common path elements, the beam is split into its different wavelength components. One of the wavelength components is not modulated and is used as a reference. The sensor typically modulates the remaining wavelength components, however, to include information representative of the parameter of interest.
Thus, each modulated wavelength component is proportional to the parameter of interest. Because the modulated components are subject to the common mode losses, however, the losses will also influence the determination of the parameter of interest if a modulated component is used, by itself, to monitor the parameter.
Fortunately, the ratio of each modulated wavelength component over the unmodulated or reference wavelength component is also proportional to the parameter of interest. Because the modulated and reference wavelength components of the beam each traverse the common path elements whose losses are to be accounted for, the optical losses introduced by the elements are the same for each wavelength component. As a result, if the ratio of the modulated and reference wavelengths is used to evaluate the parameter of interest, the losses effectively cancel and do not influence the evaluation.
While the use of multiple wavelengths does allow common mode losses to be removed, the operation of the sensor may still be subject to "differential-mode" losses attributable to variations in the optical paths traversed by the different wavelength components once the input beam is split. Because the optical losses associated with these differential paths typically are not the same, the losses do not cancel when the ratio of modulated to reference wavelength components is used to monitor the parameter of interest. These differential-mode losses can be measured and accounted for electronically, as described above, but the process is expensive and time-consuming.
Another type of optical sensor in which some form of initial calibration may be important is a multiple-phase optical sensor. Such a sensor typically responds to a parameter of interest, like position, by producing two outputs exhibiting a relative phase difference. The analysis of this phase difference allows, for example, the direction of motion to be determined and position to be determined with higher resolution via interpolation.
To produce meaningful relative phase information, however, the various components of the sensor must be constructed and aligned to exacting tolerances. As a result, conventional multiple-phase sensors have been relatively difficult and expensive to produce. The prior art has suggested the use of adjustable lenses, mirrors and apertures to alter the optical paths within the sensor to introduce the desired relative phasing between the paths and otherwise eliminate the need for initial phase calibration.
One particular scenario of interest in which sensor losses and constructional variations present a problem relates to sensor interchangeability. In many applications, it is sometimes necessary to replace a damaged, inoperative, or outdated sensor. As will be appreciated, if the various optical components of a sensor are constructed and assembled to extremely close tolerances, the optical characteristics of the sensor will also fall within a relatively narrow range. As a result, an old sensor can be replaced with a new sensor, without significantly altering the operation of the sensor. The parameter of interest can then be evaluated by applying the new sensor's output to the same formula or look-up table used with the replaced sensor.
Unfortunately, it is often prohibitively expensive to produce optical sensors within such close tolerances. As a result, the performance of a new sensor may be considerably different than the that of the old sensor. As will be appreciated, these sensor-to-sensor variations can be accounted for by actually measuring the characteristics of the new sensor and altering the formula or look-up table used to compute the parameter of interest. Unfortunately, such a recalibration procedure is time-consuming, inconvenient, and expensive.
The problems presented by the recalibration of a system for use with a new sensor are particularly acute when only a portion of a complete sensing system is replaced. For example, some sensors do not include light sources or detectors and, instead, modulate optical beams received from remote sources and transmit the modulated beams back to remote detectors. The recalibration performed if such a sensor is replaced in a system is complicated by the need for information about both the sensor and the remote sources and detectors.
Reviewing now one particular prior an arrangement, U.S. Pat. No. 4,672,201 (Welker) discloses a multiple-phase, distance-measurement sensor that includes a plurality of light source and light detector pairs. An opaque scale and grating with transparent graduations are positioned between the sources and detectors and are movable relative to each other. Light directing lenses, positioned adjacent the light sources, can be adjusted in one direction to control the relative phase of the various detector outputs and in another direction to control the relative gain of the outputs. Thus, independent control over phase and gain is provided.
The Welker patent, however, nowhere addresses the problem of sensor interchangeability. The adjustments to gain are made simply to control the relative response of the different source/detector pairs in the sensor. In addition, because the Welker sensor includes light sources and detectors, the potential problems that might be presented by the replacement of a sensor used with remote sources and detectors are reduced.
In view of these observations, it would be desirable to provide a relatively low-cost sensor that can be used to quickly replace an existing sensor, without altering the way in which the sensor's output is processed.